There are a variety of paper applications which benefit from increased paper dry and wet strength. Dry strength is particularly desirable for applications such as high performance liner board, tissue and towel, printing and writing grades, and for improving the runnability of newsprint. Major application areas for wet strength include liquid packaging board and towels.
Dry strength, generally measured by tensile strength, burst strength or compression strength, can be increased by utilizing increased amounts of fiber, by utilizing a higher proportion of long fibers, or by use of various natural and synthetic polymeric additives such as starch, vegetable gums, acrylamide polymers and polyvinyl alcohol. Generally it is found that the most economical method is use of polymeric additives.
The wet strength of ordinary paper, defined as the resistance of the paper to rupture or disintegration when it is wetted with water, is only about 5% of its dry strength. A variety of paper treatments has been proposed in the art to increase the wet strength of paper. The most widely practiced method commercially is the incorporation of wet strength resins. These are either of the "permanent" or "temporary" type, defined by how long the paper retains its wet strength after wetting with water. While high wet strength is desirable in many applications, papers having such characteristics are often repulpable only under severe conditions. While there are resins that provide temporary wet strength and thus better repulpability, their wet strength is generally not as high as that obtained with the so called permanent wet strength resins. Many wet strength resins also increase the dry strength of the paper in which they are incorporated.
Polyaminoamide-epichlorohydrin resins, which contain azetidinium functionality, are widely used commercially for wet strength. Resins containing azetidinium groups are substantive and thus self-retaining to cellulose fibers. Such resins undergo crosslinking upon a rise in pH, loss of water and/or increase in temperature to form an insoluble polyamide network. This type of resin produces quite substantial increases in dry and wet tensile strength. Resins containing azetidinium groups are available commercially from Hercules Incorporated, Wilmington, Del. as Kymene.RTM.557H, Kymene.RTM.557LX and Kymene.RTM.736 wet strength resins.
The high levels of "permanent" wet strength that are produced by resins containing azetidinium functionality is generally accompanied by difficulty in recycling or reclaiming the paper by repulping back to individual fibers. Repulping such paper requires treating it under heat and chemical conditions adequate to cause amide hydrolysis while subjecting it to sufficient physical forces to break apart the fiber network. The most common approach is the use of alkali, but oxidants, e.g., hypochlorite or persulfate, are often employed also.
U.S. Pat. No. 5,466,337 describes repulpable wet strength paper containing a permanent cationic wet strength resin and a temporary cationic wet strength resin, the temporary cationic wet strength resin being a glyoxylated vinylamide wet strength resin, and the permanent wet strength resin being selected from the group consisting of polyamine epichlorohydrin, polyamide epichlorohydrin, and polyamine-amide epichlorohydrin resins.
U.S. Pat. No. 5,585,456 discloses wet strength resins prepared by reaction of a dialdehyde and epichlorohydrin with polyaminoamide prepared from a polyamine and polycarboxylic acid or ester. Wet strengthened paper are stated to be more easily repulped than paper made with conventional wet strength agents.
U.S. Pat. Nos. 3,700,623 and 3,772,076 disclose a copolymer of acrylamide and diallylamine which is reacted with epichlorohydrin. The product imparts enhanced wet and dry strength to paper.
U.S. Pat. No. 3,372,086 describes a cationic water-soluble resin produced by reacting a polyalkylenepolyamine with a saturated aliphatic dialdehyde and then with epihalohydrin. The resin effects increased wet strength in paper.
There is continuing pressure on the paper industry by environmentally conscious paper users to increase the repulpability of paper products. Moreover, stricter legislative standards are being imposed on the paper industry. As indicated above, recycling is difficult for some papers containing strength resins. Consequently, there is a need for new polyazetidinium resins that can impart substantial wet and dry strength to paper while improving the repulpability of the paper as compared to conventional wet strength resins.